Dr. Terry Diamond and colleagues of St. George’s Hospital in New South Wales just published the first head-to-head comparison of 5,000 IU/day to 2,000 IU/day. Remember, the Food and Nutrition Board says 4,000 IU/day is the upper limit, but Dr. Diamond knows the pharmacology of vitamin D well enough to know that quite a few people will still have inadequate levels at 4,000 IU/day.

He recruited 30 patients with vitamin D levels less than 20 ng/ml and put half on 5,000 IU/day and half on 2,000 IU/day for three months. He measured a number of things, the most important of which was muscle strength.

After 3 months of 2,000 IU/day the vitamin D levels averaged 30 ng/ml (75 nmol/L), meaning about half the patients were still vitamin D deficient. Not so with the 5,000 IU/day group. The average vitamin D level was 45 ng/ml (114 nmol/L), right in the “natural range.” In addition, 93% of the patients had levels higher than 30 ng/ml compared to the 2,000 IU/day group, where only 45 % had levels above 30 ng/ml. Remember, one of the problems with daily dosing is that you must rely on the patient to take their medication. As an old GP, I am here to tell you not all patients take their meds; the ones that get me are the ones who look me straight in the eye and tell me something I know is not true.

In Dr. Diamond’s well-designed study, changes in grip strength compared to baseline were very significant, while the improvements in timed tests of sitting to standing and the 6-meter walk test also improved, but not significantly. What surprised me was that the improvements did not vary with dosage. That is, the 2,000 IU/day had the same improvements in grip strength as did the 5,000 IU/day, meaning muscle strength improvements are the most dramatic at changes in lower ranges of vitamin D levels. By that, I mean if your level is 5 ng/ml to start out and you get to up to 20 ng/ml, your percentage improvement in muscle strength will be much more dramatic than someone who went from 20 to 35 ng/ml.

I am glad to see Australians using daily dosing of vitamin D. Many of the “Stoss” doses, 100,000 IU/month or 600,000/year are not physiological, and are dangerous. Vitamin D was made every day in the skin of our ancestors and we should strive to replicate such dosing schedules. How much do we need? To quote Dr. Diamond, “This study demonstrates that the administration of oral vitamin D at 5,000 IU daily is superior to 2,000 IU daily for 3 months to treat mild to moderate vitamin D deficiency.”